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From: "dimitri.gorokhovik at free dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/94485] New: [10.0.1, c++2a] g++ optimizes away the code,
 accepts arbitrary inline asm
Date: Sat, 04 Apr 2020 15:33:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94485

            Bug ID: 94485
           Summary: [10.0.1, c++2a] g++ optimizes away the code, accepts
                    arbitrary inline asm
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dimitri.gorokhovik at free dot fr
  Target Milestone: ---

Created attachment 48194
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D48194&action=3Dedit
Sample code illustrating the sighting.

Version: gcc (GCC) 10.0.1 20200404 (experimental)

command line:
~/gcc-trunk/dist/bin/g++ -std=3Dc++2a  -O3 -Wall -Wextra -W bug-20200404170=
9.cpp
-o bug-1 && ./bug-1


The attached file (apologies, still big) contains the following code (line
425):

  constexpr iterator (tesselation const& me)
   : cube_ { me.cube_ },
     inner_ { me.as_parent ().begin () },
     tess_ {},
     outer_ { tess_ }
  {
   if (inner_)
   {
     tess_ =3D tesselation_of_two_cubes { cube_, *inner_ };
     asm("before:");
#if 0 // breaks only with -O3
     auto const tmp { tess_.begin () };
     asm("middle:");
     outer_ =3D tmp;
     ^^^^^^^^^^^^
#else // breaks with -O3, -O1, fixes with -fsanitize=3Dundefined|null
     outer_ =3D { tess_.begin () };
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     asm("after:");
#endif
     inner_.next ();
     if (! outer_) throw 749;
   };
  };

(Minor remark: 'asm' statements in this constexpr method raise no brows. I
tried putting there insns that require evaluation, seems anything is silent=
ly
accepted. The same asm statements are rejected in a more simple constexpr
code.)

It seems, some optimization levels completely remove the highlighted assign=
ment
so that 'throw 749' is executed (main() prints it then). Other passes keep =
the
copying code and produce expected result. This over-optimization can be fix=
ed
by enabling certain sanitizers: undefined, null, vptr.

The asm code produced:

-- broken (compiled with -O3 or -O1):
#APP
# 425 "bug-1.cpp" 1
        before:
# 0 "" 2
#NO_APP
        movq    %r12, 216(%rsp)
#APP
# 433 "bug-1.cpp" 1
        after:
# 0 "" 2
#NO_APP

-- working (compiled with -O0 or -O1 or with -fsanitizer=3Dnull):
#APP
# 425 "bug-1.cpp" 1
        before:
# 0 "" 2
#NO_APP
        xorl    %eax, %eax
        cmpb    $0, 43(%rbx)
        movq    %r14, 64(%rsp)
        movw    %ax, 72(%rsp)
        movb    $0, 74(%rsp)
        je      .L73
        cmpb    $0, 63(%rbx)
        je      .L73
        cmpb    $0, 83(%rbx)
        sete    %al
.L68:
        movb    %al, 75(%rsp)
        movq    64(%rsp), %rax
        movdqa  64(%rsp), %xmm6
        movq    %rax, 88(%rbx)
        movl    72(%rsp), %eax
        movaps  %xmm6, 96(%rsp)
        movl    %eax, 96(%rbx)
#APP
# 433 "bug-1.cpp" 1
        after:
# 0 "" 2
#NO_APP=